As a general rule, the friction devices in power transmissions are supplied with lube and cooling flow continuously from fluid which has passed through bushings or bearings. Controlled lube flow has been provided by using computer controlled lube flow circuits and valves or by establishing a valving relation between the actuating piston of the friction device in a shaft member.
The valving relation provides cooling flow when the device is engaged due to movement of the piston and requires closely held tolerances on the piston and shaft to control the proper timing of the delivery. This adds to the cost of manufacture. The wear normally associated with the friction device, which affects piston movement, can complicate this drawback. The use of computer controls adds to the manufacturing and operating cost and requires additional valving to control the flow. This valving is generally electrically activated and further adds to the cost of the system.